With Iran's conservative camp divided, reformists were confident of a good showing by the moderate cleric they back in Friday's presidential vote as they seek to avenge the disputed re-election of Mahmoud Ahmadinejad four years ago.

Thanks to a late surge of support, cleric Hassan Rowhani has emerged as one of the front-runners in the poll and has a real chance of forcing a run-off against the conservatives, analysts say.

The support came after the moderate and reformist camps this week joined ranks behind 64-year-old Rowhani after pressurising the sole reformist candidate, Mohammad Reza Aref, to withdraw from the race.

Infographic: Who will be Iran's new president? (Design by Farwa Rizwan/Al Arabiya English)

Sources said behind-the-scenes negotiations were underway in the conservative camp on Thursday in a last-ditch attempt to unify ranks in a bid to beat back Rowhani's challenge.

On Wednesday, efforts were reportedly made to persuade three of the five conservative candidates in the six-horse race to withdraw in favour of those shown to be leading the field -- Qalibaf and Jalili.

However, the conservative contenders dismissed any suggestion of withdrawing, ensuring that a total of six candidates will line up on Friday, reports said.

Rowhani's backers were on Thursday working the phones and social media networks to urge a massive turnout in favour of the moderate-level cleric, who was Iran's chief nuclear negotiator from 2003 until 2005.

The unified support for Rowhani does not mean that reformists are united but it does boost his chances of forcing a run-off, according to Tehran-based conservative political analyst Mehdi Fazayeli.

Rowhani “is now regarded as one of the leading candidates and (his) chances of being in the second round, if there is one, are more evident,” said Fazayeli.

The managing editor of the hardline Kayhan newspaper warned on Thursday said that the conservative candidates' stubbornness may “transform the river of conservative votes into a multitude of small streams.”

Reza Marashi of the US-based National Iranian American Council advocacy group, said it was vital for the conservatives to rally behind a single candidate.

“Unless the remaining conservative candidates coalesce, a splintered conservative field will face a candidate in Rowhani who has the backing of significant political and social forces in Iran,” Marashi told AFP.

Rowhani has received the endorsement of ex-presidents Akbar Hashemi Rafsanjani, a pragmatist, and the reformist Mohammad Khatami.

“I ask everyone, especially reformists” to vote for Rowhani, said Khatami, who was president between 1997 and 2005.

Rafsanjani, a pillar of the Islamic republic who was in power between 1989 and 1997, too voiced his support for Rowhani, who also served him.

Rafsanjani urged voters Wednesday to vote despite “doubts,” adding, according to the Mehr news agency, that “surveys show that Mr. Rowhani is ahead”.

Four years ago, the reformists' hopes of returning to power were dashed by the disputed re-election of Ahmadinejad.

The two unsuccessful reformist candidates, Mir Hossein Mousavi and Mehdi Karroubi, immediately alleged fraud and called on their supporters to demonstrate.

The protests turned deadly and were crushed after some months by the security forces. Mousavi and Karroubi have been under house arrest since 2011.

Some 50.5 million voters are eligible to vote on Friday for a successor to Ahmadinejad, under whose presidency Iran has been isolated internationally over its controversial nuclear drive.

International sanctions slapped on Tehran in a bid to force it to give up its sensitive enrichment work have sparked a deep economic crisis in the Islamic republic, which has been the focus of the election campaign.

Only eight male candidates out of nearly 700 registrants were approved by the hardline Guardians Council, Iran's electoral watchdog, to run in the race. Two subsequently dropped out.